INVESTIGATION OF BIO-INSPIRED MICRO-TEXTURED SURFACE FOR DRAG REDUCTION

Abstract

Bio-inspired surfaces have been a relevant field of study in recent years. Riblets are surface structures inspired by the skin of sharks. Despite having a considerable weight, sharks have excellent maneuverability. This work deals with the concept of drag reduction from a holistic view. There are various examples of engineering applications that could be inspired from nature. Drag produced over navigating objects such as aircraft, ships, etc., plays a vital role in navigation economics. Experiments have confirmed that minor alterations in the viscous or buffer layer could affect the wall shear drag produced by the turbulent boundary layer. Literature shows the efficacy of riblets in altering drag behavior, but the physical mechanisms involved with this effect have not yet been fully understood in scientific terms. Biomimetic structures inspired by sharkskin, bird feathers, and conventional shapes could significantly reduce drag. Drag over aerial vehicles affects their aerodynamic characteristics. Detail study on wing structure would improve performance and enable higher precision control over aerial vehicles. Making the aerodynamic structure flexible is one approach towards mimicking the naturally occurring wings. The structures of birds and aquatic animals have a flexible design to impart better maneuverability. In the present study, bio-inspired modifications are analysed for drag reduction purposes. Numerical gives a better understanding of flow mechanisms and allows us to design riblets based on the specific field of application. Study is done to investigate the effect of sawtooth riblets over a flat plate. The aim is to quantify the impact of skin friction on this textured surface compared to a smooth wall. The effect of geometric and flow parameters on riblets is analysed using numerical simulation. The interactions of the overlying turbulent flow with the riblets are investigated. The riblets show a reduction in the velocity and vorticity near the wall. The result indicates that the riblet tip imparts a movement to the streamwise longitudinal vortices away from the wall, thus alleviating Reynolds stress and weakening turbulence flow characteristics in the near-wall region. The study would help optimize the riblet design based on the flow conditions. The correlation of drag reduction with non-dimensional parameters is shown and validated for different flow conditions. The optimum result is found at the s+ range of 15-20, with the spacing/height ratio as two, independent of the Reynolds number range and velocity studied. A maximum drag reduction of 9.46% is achieved.Based on the numerical study, an experimental setup is fabricated to perform the drag reduction experiments using external flow conditions over a flat surface, a torpedo model, and an aerofoil profile. Different modified surface structures (riblets) are used to investigate the flow alteration and the consequent drag modification using these surfaces. The experiments done on flat surfaces observed a maximum reduction of 13.2% in shear drag. Mechanisms of fluid drag in turbulent flow and riblet drag reduction theories are discussed. Detailed flow field and force measurements are done using PIV (Particle Image Velocimetry), CTA (Constant Temperature Anemometry), flow visualization, and Shear stress probe. The experiments are performed in a subsonic wind tunnel for the Reynolds numbers (Re) covering the laminar-turbulent transition range. The study over torpedo reveals that the riblets enhance the momentum flux, thus enabling the surface to reduce drag by suppressing the vorticity generation in the wake and reducing the turbulent dissipation. A study on NACA0015 aerofoil examines the impact of chordwise foil flexibility and the effect of ribbed structure on the dynamical features of flapping-based propulsion. This work explores the physical mechanisms responsible for turbulent drag reduction. A maximum drag reduction of 11.7% is achieved with only 1/6th part of the torpedo covered with riblets. The riblet profile stabilizes the flow due to the presence of longitudinal profiles and prevents crossflow motion. The riblets reduce the formation length and wake width, reducing the adverse pressure gradient zone and decreasing the power requirement. Riblets effectively reduce drag over an aerofoil, and a maximum drag reduction of 21% is observed using 1/4th surface of aerofoil covered with riblets. Flexibility imparts twice the propulsion in the flapping range studied. The results strongly suggest that the chordwise-induced flexibility increases the aerofoil deformation and, thereby, the effective flapping amplitude. The flexible structure increases the advection of transverse velocity fluctuations and viscosity diffusion, thus increasing the envelope of the velocity jet. Flexibility suppresses meandering by imparting increased convection to the shed vortex. The numerical modeling and experiments collectively explore the effect on the flow and turbulent characteristics using these structures. Conclusions concerning the holistic investigation of the drag reduction using bio-inspired structures have been drawn based on the research objectives, along with the recommendation for the future work has been put forward.